openGL - Render multiple objects using different shaders

Question

I am trying to render a number of orange triangles and superimpose a few black lines on them (so to form a nice cube). I am using two shaders to render the different colors:

an orange fragment shader (fragment.glsl) the i use to draw the triangles:

#version 120

void main() {
    gl_FragColor = vec4(0.8,0.4,0.1,1.0);
}

and a black shader that i use to draw the lines (fragment_lines.glsl):

#version 120

void main() {
    gl_FragColor = vec4(0.0,0.0,0.0,1.0);
}

The code I am using to generate the rotating cube (orange triangles + black lines) is the following:

#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include "matrix.h"
#include "shader.h"
#include "util.h"

int main(void) {
    if (!glfwInit()) {
        return -1;
    }

    GLFWwindow *window = glfwCreateWindow(800, 800, "HelloGL", NULL, NULL);
    if (!window) {
        glfwTerminate();
        return -1;
    }

    glfwMakeContextCurrent(window);
    glEnable(GL_DEPTH_TEST);

    if (glewInit() != GLEW_OK) {
        glfwTerminate();
        return -1;
    }

    GLuint program = load_program("shaders/vertex.glsl", "shaders/fragment.glsl");
    GLuint program_lines = load_program("shaders/vertex.glsl", "shaders/fragment_lines.glsl");
    GLuint position = glGetAttribLocation(program, "position");
    GLuint position_lines = glGetAttribLocation(program_lines, "position");
    GLuint matrix = glGetUniformLocation(program, "matrix");
    GLuint matrix_lines = glGetUniformLocation(program_lines, "matrix");


    float data[] = {
    -1.0f,-1.0f,-1.0f, // triangle 1 : begin
    -1.0f,-1.0f, 1.0f,
    -1.0f, 1.0f, 1.0f, // triangle 1 : end
    1.0f, 1.0f,-1.0f, // triangle 2 : begin
    -1.0f,-1.0f,-1.0f,
    -1.0f, 1.0f,-1.0f, // triangle 2 : end
    1.0f,-1.0f, 1.0f,
    -1.0f,-1.0f,-1.0f,
    1.0f,-1.0f,-1.0f,
    1.0f, 1.0f,-1.0f,
    1.0f,-1.0f,-1.0f,
    -1.0f,-1.0f,-1.0f,
    -1.0f,-1.0f,-1.0f,
    -1.0f, 1.0f, 1.0f,
    -1.0f, 1.0f,-1.0f,
    1.0f,-1.0f, 1.0f,
    -1.0f,-1.0f, 1.0f,
    -1.0f,-1.0f,-1.0f,
    -1.0f, 1.0f, 1.0f,
    -1.0f,-1.0f, 1.0f,
    1.0f,-1.0f, 1.0f,
    1.0f, 1.0f, 1.0f,
    1.0f,-1.0f,-1.0f,
    1.0f, 1.0f,-1.0f,
    1.0f,-1.0f,-1.0f,
    1.0f, 1.0f, 1.0f,
    1.0f,-1.0f, 1.0f,
    1.0f, 1.0f, 1.0f,
    1.0f, 1.0f,-1.0f,
    -1.0f, 1.0f,-1.0f,
    1.0f, 1.0f, 1.0f,
    -1.0f, 1.0f,-1.0f,
    -1.0f, 1.0f, 1.0f,
    1.0f, 1.0f, 1.0f,
    -1.0f, 1.0f, 1.0f,
    1.0f,-1.0f, 1.0f
    };

    GLuint buffer = gen_buffer(sizeof(data), data);
    GLuint buffer_lines = gen_buffer(sizeof(data), data);

    while (!glfwWindowShouldClose(window)) {

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        float mat[16];
        mat_identity(mat);
        mat_translate(mat, 0, -0.5, 0);
        mat_rotate(mat, 0.1, 0.3, 1, glfwGetTime());
        mat_ortho(mat, -2, 2, -2, 2, -2, 2);


        glUniformMatrix4fv(matrix, 1, GL_FALSE, mat);
        glUniformMatrix4fv(matrix_lines, 1, GL_FALSE, mat);

        glBindBuffer(GL_ARRAY_BUFFER, buffer);
        glEnableVertexAttribArray(position);
        glVertexAttribPointer(position, 3, GL_FLOAT, GL_FALSE, 0, 0);

        glBindBuffer(GL_ARRAY_BUFFER, buffer_lines);
        glEnableVertexAttribArray(position_lines);
        glVertexAttribPointer(position_lines, 3, GL_FLOAT, GL_FALSE, 0, 0);


        glUseProgram(program);
        glDrawArrays(GL_TRIANGLES, 0, 12*3);

        glUseProgram(program_lines);
        glDrawArrays(GL_LINES, 0, 12*3);

        glDisableVertexAttribArray(position);
        glDisableVertexAttribArray(position_lines);

        glBindBuffer(GL_ARRAY_BUFFER, 0);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

However, when I run this code I can only see the black lines on screen. If I swap the order of

glUseProgram(program);
glDrawArrays(GL_TRIANGLES, 0, 12*3);

glUseProgram(program_lines);
glDrawArrays(GL_LINES, 0, 12*3);

Then I see only the orange triangles and no sign of the black lines.

How can i set this up so that both cube faces (orange triangles) and the corresponding edges (black lines) are visible?

ps.:

I am using Ubuntu 18.04.

I know I should remove duplicate points to improve efficiency, but this is a simple example.

I know the lines coords are a bit silly, they should be good enough to see the example working.

The matrix code can be found in https://github.com/fogleman/HelloGL

Answer 1

Make two draw functions like

DrawCube()
{
 glUniformMatrix4fv(matrix, 1, GL_FALSE, mat);
 glBindBuffer(GL_ARRAY_BUFFER, buffer);
 glEnableVertexAttribArray(position);
 glVertexAttribPointer(position, 3, GL_FLOAT, GL_FALSE, 0, 0);

 glUseProgram(program);
 glDrawArrays(GL_TRIANGLES, 0, 12*3);

 glUseProgram(0);
 glBindBuffer(GL_ARRAY_BUFFER, 0);
}

The problem with your code is you are mixing shader states for two draws. Same separate and write code for DrawLines() and them in render function just call these both.